Yes, right. Composite models cannot be fit at the moment. I started working on this but am not quite done yet. They should have `param_names` and `parameters` attributes constructed from their children's. One annoying aspect of this is that there may be a name collision - children models may have a parameter with the same name. One option is to prepend the model name but this doesn't seem general enough. I'm open to any ideas on how to deal with this.